1. Field of the Invention
This invention relates to a diode package and more particularly to a diode package having a compressible dielectric member.
2. Description of the Prior Art
It is often desirable to construct a diode package which has low values of capacitance, inductance and resistance in order to meet particular electrical requirements. One such application utilizes a PIN diode that switches inductors in and out of a varactor-tunable resonator filter used in TV tuners. Diode packages having the above-mentioned electrical properties are highly desirable in such TV tuners, especially at UHF frequencies (470-890 MHz) wherein such frequencies excessive package parasitics limit the available tuning bandwidth of a varactor and high package resistance deteriorates the loss of the preselector and postselector filters of the tuner. Another such application of a diode package having such characteristics is in microwave circuits using high power TRAPATT diodes wherein effective heat dissipation is necessary for desired performance.
Many of the prior art diode packages are formed of a hollow cylindrical spacer member of dielectric material with electrically conductive contact members covering the end faces of the spacer member. A diode enclosed within such a spacer member is mounted generally with one diode terminal attached directly to one conductive contact member and with the other diode terminal connected to the other contact member by thin bonding wires or ribbon. Typical examples of this structure are shown in U.S. Pat. No. 3,787,782 entitled "Microwave Device," issued on Jan. 22, 1974, and U.S. Pat. No. 3,320,497 entitled "Variable Capacitance Diode Packages," issued on May 16, 1967. One problem with such a package structure is the high inductance and resistance resulting from the thin bonding wires or ribbon which are connected to the diode. The wire or ribbon in these packages is typically very thin to assure a good bond to the diode, the bond usually formed by spot welding or thermo-compression bonding. Some prior art structures utilize a plurality of bonding wires to reduce the high resistance which results from the thin wire but undesirable parasitic effects due to high inductances are still present. Also, the use of thin wire or ribbon to connect the diode does not contribute effectively to the dissipation of heat and allows heat transfer only through the diode terminal attached directly to the conductive contact member.
Other prior art packages are fabricated without bonding wires and utilize contact pressure to make an electrical connection between an electrically conductive contact member and the diode. One problem with this type package is that external pressure means are generally required to maintain the electrical contact. These pressure means contribute to the complexity of the package and hence its cost. However, and of more significance, pressure contacts are not reliable in assuring a good electrical connection. Air gaps and voids which may be caused by nonuniform distribution of pressure or uneven material finishes at the interfacing surfaces create undesirable parasitic effects which degrade performance of the device. Severe variations in temperature may tend to reduce the contact pressure which may also result in reduced contact area, or, possibly, in an open circuit.
Another desirable quality of such diode packages in addition to the electrical properties is the cost of fabrication. Simplicity of construction is one important factor for low cost fabrication. The bonding of wires or ribbons to diodes in the prior art arrangements is generally a time-consuming operation and often very difficult requiring highly skilled operators or costly equipment or both. In addition to the desirable simplicity of structure, another cost factor is the capability of making diode packages in quantity. Many of the prior art packages utilize ceramic beryllia (BeO) or alumina (Al.sub.2 O.sub.3 ) as the dielectric material for the insulating spacer member. These ceramics have excellent electrical insulating characteristics and fairly low dielectric constants (.epsilon..sub.r .perspectiveto. 10 ) for minimizing the package capacitance and have good heat conducting properties, especially beryllia. However, these ceramics are not desirable for cutting or slicing operations useful in producing diode packages in quantity. The cutting or slicing of beryllia requires special tools and equipment since the dust which may be produced is toxic. Both alumina and beryllia are very brittle and cutting or slicing operations would tend to result in a high breakage rate making these materials undesirable for such cutting or slicing processes.